Field of the Invention
The present invention relates to a magnetic disk apparatus and a magnetic disk which is used in the magnetic disk apparatus.
In recent years, a significance of a magnetic disk apparatus as an external memory device of a computer system has been more and more increased and its recording density has remarkably been improved every year.
It is better to reduce a distance between a magnetic head and a magnetic layer of a magnetic disk (hereinafter referred as "spacing") upon recording/reproduction in order to improve the recording density of the magnetic disk apparatus. To reduce the spacing, a distance between a magnetic head and a surface of a magnetic disk (hereinafter referred as "flying height") is also reduced. To assure the floating stability of the magnetic head in such a case, it is required that the surface of a magnetic disk is as flat as possible.
On the other hand, upon start and stop of the magnetic disk apparatus, a frictional force which is generated between the magnetic head and the magnetic disk causes an abrasion between them and becomes a cause or deterioration of characteristics such as writing characteristic, reading characteristic, and the like. Further, if a moisture or the like exists between the magnetic head and the magnetic disk in a stop state of the magnetic disk, both of them are strongly adsorbed. If the apparatus is started in such a state, a large frictional force occurs between the magnetic head and the magnetic disk and there is a fear such that the magnetic head and the magnetic disk are damaged. There is a tendency such that the above frictional force increases as the surface of the magnetic disk is flat. Such a tendency is contradictory to the requirement for the floating stability of the magnetic head in association with the improvement of the recording density mentioned above.
There has been known a method whereby micro protrusions are formed on the surface of the magnetic disk in order to reduce such a frictional force.
In JP-A-1-134720, for instance, there has been disclosed a method whereby island-like projections are formed on the surface of a magnetic disk.
In JP-A-1-122028, there has been disclosed a method whereby a metal alkoxide solution is coated onto the surface of a magnetic layer of a magnetic disk and is rapidly heated and a protective layer having projections is formed onto the surface of the magnetic layer.
In JP-A-57-20925, there has been disclosed a method whereby cylindrical projections each having a diameter of 0.03 to 0.1 mm and a height of about 0.05 .mu.m are formed onto the surface of a magnetic layer or a protective layer.
All of the above conventional techniques intend to reduce a flying height of the magnetic head for the magnetic disk and to prevent that, in such a case, a slider to support the magnetic head is adsorbed to the magnetic disk.
In the above conventional techniques, nothing is considered with respect to a point that the floating stability of the magnetic head is continued for a long time.
Further, nothing is considered with respect to a point that a change of a spacing is caused by a change of flying height of the magnetic head due to a difference of a radial position of the magnetic disk, which limits the recording density.